Rapid Voltammetric Determination of Traces of Zn2+ in 105 Excess of Ni2

The impurities determination in Ni electrolysis baths is of a great importance for the quality of the Ni electroplating protective and decorative coatings. Inclusions having more negative potential than Ni form corrosion anodic zones in the presence of humidity, corrupting the Ni layer protective properties. One of the most dangerous inclusions is Zn.

Zn often appears in the Ni coatings deposited on brass, as a result of the brass detail dissolution in the low pH Ni baths before the beginning of the electrolysis: the deposition of the first Ni layer.

As known, the maintenance of the Zn²⁺ concentration in the Ni bath bellow 3 ppm can prevent Zn inclusions formation in the Ni coatings, which requires on-line Zn²⁺ determination. Nevertheless, spectral analytical methods such as AAS and ICP cannot be applied after Zn²⁺ extraction from the high concentrated matrix, because the huge Ni²⁺ concentration will provoke crystallization in the nebulizer area corrupting the analysis. The direct application of some voltammetric techniques also cannot yield any results, because of the very small difference of the E_½ of the two species, which will cause peak overlapping. Some specific properties of the Ni²⁺ and Zn²⁺ ions however can be used to avoid the interferences. It is known that Zn²⁺ forms soluble hydroxyl complex in the presence of excess of OH^-, while Ni²⁺ forms the very stable Ni(OH)₂ precipitation product. Thus, KOH or NaOH can be applied as a supporting electrolyte, in which a well defined peak of Zn²⁺ is registered as shown in Fig. 1, while no peak of Ni²⁺will appear because of its precipitation.

The effect of the NaOH concentration applied as a supporting electrolyte, of the Zn²⁺ adsorption by the formed Ni(OH)₂ precipitation product, and of the Ni²⁺/Zn²⁺ concentration ratio on the analytical characteristics of the Zn²⁺ determination are the subject of this study. The results are presented and discussed. It is demonstrated that the developed method consisting in adding of a small volume of Ni electroplating bath solution to the NaOH supporting electrolyte, followed by DPV determination can be applied for the on-line Zn²⁺ determination.